1. Field of the Invention
The invention relates to nano-scale structures and, more specifically, to planar field emitters.
2. Description of the Prior Art
Cold cathode field emission occurs when the local electric field at the surface of a conductor approaches about 109V/m. In this field regime, the work function barrier is reduced enough to permit electronic tunneling from the conductor to vacuum, even at low temperatures. To achieve the high local fields at experimentally achievable macroscopic fields, field emission sources are typically made from sharp objects such as etched wires, micro-fabricated cones or nanostructured conductors such as carbon nanotubes (CNTs).For the majority of field emission applications, the cathode current needs to be controllable. In general, control is achieved with a gate located nearby the field emission source that generates the field used to eject electrons from the field emission source but only absorbs a fraction of the emitter current.
Cold cathode field emission devices have the capability to produce very high current density electron beams (greater than 100 A/cm2) with low power consumption. However field emission devices have not, to date, been incorporated into commercial high current density applications such as power microwave electronics because field emission sources may fail prematurely unless extreme care is taken to protect the devices.
Typical field emission devices are variants of the conventional Spindt field emission array. This device design has several inherent vulnerabilities stemming from the small dimensions required to achieve a high enough field strength to emit electrons from a conical structure. Under ideal operating conditions (e.g. 10−9 Torr, with no perturbation in the gate voltage, gate currents or anode voltage), Spindt emitter arrays have been shown to emit in excess of 40 A/cm2 for extended periods of time. In most applications however, the electron source typically encounters occasional plasma discharges, called spits. Spits are often caused by gas desorption from an anode surface that is ionized by the electron beam. The resulting plasma generates an arc between the anode and nearby surfaces at a lower potential such as the field emitter. Depending upon the cable capacitance, potential difference and embedded circuit protection, a spit has the potential to destroy field emitter devices, even if the spit does not land on the device itself. In high voltage applications, such as x-ray tubes, because spits typically draw more than 100 amps for less than 1 microsecond, the inductively and capacitively coupled currents will often destroy Spindt field emitter devices, even if the spit does not directly impact the field emission source. In addition, during the spit, the voltage on the anode often drops to a low enough value that the anode is no longer able to absorb the cathode current. Therefore, the gate electrode absorbs up to the entire cathode current. At moderate current densities in Spindt emitters, (greater than about 100 mA/cm2), localized heating from the excessive gate current can destroy the device quickly.
Recently, nanostructured materials, such as carbon nanotubes, have been proposed as field emission sources. Because of their narrow diameter, high electrical conductivity and high thermal conductivity they offer the potential for field emission sources that operate at lower gate voltages compared to conical emitters. To date however, nanostructured field emission sources have not achieved current densities demonstrated in Spindt field emission source.
Therefore, there is a need for a field emission source capable of producing high current density that is more robust than conventional Spindt field emission devices.
There is also a need for a robust field emission device in which the gate current, threshold voltage and switching speed are comparable to conventional Spindt field emitter arrays.